There are two types of the X-ray CT apparatus; a single-slice X-ray CT apparatus for obtaining one tomographic image by one-time X-ray exposure, and a multi-slice X-ray CT apparatus being able to obtain multiple tomographic images simultaneously.
The single-slice X-ray CT apparatus employs an X-ray detector in which numerous X-ray detector elements are arranged in one line, i.e., in one-dimensional direction (channel direction), so as to irradiate a fan beam, i.e., a fan-shaped X-ray beam on an object from an X-ray tube, and measure the X-ray that has passed through the object to obtain projection data of the object.
On the other hand, the multi-slice X-ray CT apparatus irradiates a corn beam, i.e., a conical or a pyramidal X-ray beam from an X-ray tube, and an X-ray detector in which numerous X-ray detector elements are arranged in two-dimensional directions (the channel direction and row direction) measures the X-ray that has passed through the object to obtain projection data of the object.
In either type of the X-ray CT apparatus, the X-ray tube and the X-ray detector opposed to each other are rotated around the object, so as to collect projection data from multiple directions. The projection data being collected is subjected to a reconstruction filtering process for deblurring, and it is further subjected to back projection to reconstruct a tomographic image of the object.
The projection data is collected at discrete locations (hereinafter, each referred to as a “view”) of the X-ray tube, and this collected projection data is referred to as “projection data at the view”. The number of views per rotation of the X-ray tube and the X-ray detector for going around the object generally extends from several hundred to several thousand. An action for collecting the projection data of the number of views necessary for reconstructing one piece of CT image is referred to as “scan”. The projection data corresponding to one view is made up of data in association with the number of channels times the number of rows of the X-ray detector described above (in the case of the single-slice X-ray CT apparatus, the number of row is equal to 1 as described above).
In the X-ray CT apparatus as discussed above, in order to identify a diagnostic object within the tomographic image being obtained, a contrast to noise ratio (Contrast to Noise Ratio, hereinafter, described as “CNR”) serves as a significant image quality index. The CNR is a value obtained by dividing an absolute value of CT value difference between the diagnostic object and its surroundings, by an image noise standard deviation value (hereinafter, described as “image SD (Standard Deviation) value”.
As for the aforementioned image quality index of the X-ray CT apparatus, increasing of exposure dose generally improves the CNR, and produces a good-quality image as an image for diagnosis. On the other hand, it is desirable to reduce to a minimum the exposure dose to a patient. Considering the situation above, the patent document 1 discloses an X-ray CT apparatus which improves the CNR in the reconstructed image by using a tube voltage (a voltage applied between anode and cathode of the X-ray tube) lower than conventionally used voltage, while not increasing the exposure dose and the image SD value, or which reduces the exposure dose by using the tube voltage lower than before, while not deteriorating the CNR in the reconstructed image.
[Patent Document 1]
Japanese Unexamined Patent Application Publication No. 2004-073865